The present invention is related to a method of broad band high resolution spectroscopic analysis of a sample and to a spectroscopic device being adapted for implementing the method.
The frequency spectrum of a periodic sequence of ultra-short light pulses consists of a regular arrangement of sharp lines (so-called frequency comb). Generally, the frequency width of the spectrum will be inversely proportional to the temporal width of the pulse envelope. Accordingly, a periodic sequence of femtosecond pulses is characterized by broadband frequency combs. The femtosecond frequency comb (see e.g. J. Reichert et al. in “Physical Review Letters” vol. 84, 2000, p. 3232, and S. T. Cundiff et al. in “Reviews of Moderns Physics” vol. 75, 2003, p. 325) has served as a universal optical clockwork mechanism (S. A. Diddams et al. in “Science” vol. 293, 2001, p. 825), has enabled new precision test of fundamental theories (M. Fischer et al. in “Physical Review Letters” vol. 92, 2004, 230802) and allows complete control of the electric field in ultra short pulses (A. Baltuska et al. in “Nature” vol. 421, 2003, p. 611). Furthermore, the combination of high peak power in femtosecond pulses and the high spectral quality of each of the lines of the frequency comb can be utilized to create narrow bandwidth sources for high resolution spectroscopy in, with that respect, previously inaccessible wavelength ranges.
As a particular application of high resolution spectroscopy, so-called broad band cavity enhanced absorption spectroscopy methods (CEAS methods) have been developed (see e.g. M. J. Thorpe et al. in “Science” vol. 311, 2006, p. 1595, and T. Gherman et al. in “Optics Express” vol. 10, 2002, p. 1033). A femtosecond pulse train generated with a mode-locked laser source is coupled into a resonator cavity including a sample to be investigated. The femtosecond pulse train and the resonator cavity are tuned such that cavity modes essentially correspond to the comb components of the femtosecond pulse train. Due to the large number of round trips of the frequency components matched to the cavity modes, the interaction of the sample with light is essentially increased. Intensity of light pulses transmitted by the resonator cavity is evaluated for obtaining a spectral absorption of the sample. The power throughput through the resonator cavity is dramatically increased as the coherent frequency comb is used as a laser source instead of an incoherent broadband source, so that the sensitivity of the broadband cavity enhanced absorption spectroscopy is essentially increased.
According to the technique of M. J. Thorpe et al., the transmitted light pulses through the resonator cavity are spectrally resolved and detected with a CCD sensor. For avoiding a source noise introduced into the transmitted pulses, a so-called ring down signal is measured after step-wise blocking the pulses coupled into the resonator cavity. For measuring the ring down signal, a scanning mirror is used for streaking the spectrally resolved light pulses to different portions of the sensitive area of the CCD sensor.
The conventional CEAS methods have a disadvantage as the resolution of these methods is limited by the spectrometer, recording the ring down signal and ignoring the high temporal coherence of the driving signal. In particular, with the conventional CEAS methods it is impossible to resolve the individual comb components of a light pulse train coupled into the resonator cavity. Resolving the comb components of a femtosecond pulse train would require a spectral disperser having an impractical dimension of about e.g. 30 cm for a repetition frequency of the pulse train of 1 GHz and associated optical components having essential drawbacks in terms of aberrations.
Combining high sensitivity of the conventional CEAS method of M. J. Thorpe et al. with high spectral resolution in an efficient and unambiguous manner proves challenging. Application of a further high resolution spectral disperser is excluded by the requirement of conventional detecting the ring-down signal with sufficient time resolution. Furthermore, for a reduction in transmission through a cavity to be interpreted as an additional loss inside the resonator, it is required that the frequency comb mode is always on resonance. This condition can in general only be achieved for two modes from a frequency comb as dispersion inside the resonator will render the frequency spacing of the modes in the resonator non-equidistant. Even if the resonator is engineered to have an equidistant mode spacing within the bandwidth of interest, adding an absorber (dielectric medium) into the resonator will make the resonant mode positions move in frequency. Therefore it is difficult to distinguish between a reduction in transmission due to dispersion or due to absorption. In the technique of T. Gherman et al. this ambiguity is resolved by dithering the resonator around the resonant frequency and measuring the time averaged transmission signal, effectively removing the dispersion effect from the result. A. Schliesser et al. (“Optics Express” vol. 14, 2006, p. 5975) have demonstrated a scheme which remedies this ambiguity, yielding both absorption and dispersion information. However, both of the latter techniques are not capable to detect the comb components with sufficient spectral resolution.
The objective of the invention is to provide an improved method of spectroscopic analysis of a sample avoiding the disadvantages of the conventional methods. Furthermore, the objective of the invention is to provide an improved spectroscopic device avoiding the disadvantages of the conventional devices.